Implement region format for TimeZoneFormat

[nordzilla]

The UTS-35 spec specifies regionFormat which is used in generic-location format, as well as some fallback formats for non-location formats.

We currently use this in the main formatting paths, but not any of the fallback formatting paths.

We should pull this out into its own piece of functionality so that it can be used in main formatting paths as well as fallbacks.

[sffc]

@nordzilla Can you clarify what is the expectation from this issue? We already support generic location format. What else needs to be done?